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Teaching a Graphics Class with OpenGL: Two Approaches

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  1. Teaching a Graphics Class with OpenGL: Two Approaches Ed Angel University of New Mexico F.S. Hill University of Massachusetts

  2. Who are we? • Ed Angel: Presidential Teaching Fellow, UNM, author of Interactive Computer Graphics (Second Edition) • Sandy Hill, UMass, NTU. Author of Computer Graphics with OpenGL

  3. Why are we doing this? • OpenGL is the dominant graphics API • Supported on almost all architectures • Makes it easy to teach a programming-oriented class • We believe OpenGL based classes are better but there are multiple approaches • Many (potential) instructors have little or no experience with OpenGL

  4. What can we do in two hours? • Try to convince you why our approaches should be considered • Intro to OpenGL • Look at two different courses that use OpenGL • Identify resources • Future trends

  5. Outline • OpenGL basics • Angel’s Approach • Hill’s Approach • Demo code/assignments • Question/answer/discussion

  6. Approaches to teaching CG • Bottom Up • Survey • Top Down • Really Top Down

  7. Kemeny’s Argument • You don’t need to know what’s under the hood to be literate, but unless you know how to program, you’ll be sitting in the back seat instead of driving. • Bottom up = study the engine • Survey = hire a chauffeur • Top Down = learn to drive

  8. What is important? • Three dimensional concepts • Light-material interactions • Transformations • Modeling • Interaction • Rendering/ Algorithms

  9. What Is OpenGL? • Graphics rendering API • high-quality color images composed of geometric and image primitives • window system independent • operating system independent • close to hardware • leads to discussions of algorithms and implementation

  10. Simple Demo • This demo shows OpenGL capabilities • Created for SIGGRAPH tutorial • Could be written by student by end of semester • Demonstrates • lighting • material properties • texture mapping • interaction (via GLUT)

  11. Physical Approaches to Rendering • Ray tracing • Close to physics for simple models • Easy to teach • Easy to start with • Limited • Radiosity • Closer to rendering equation • Not suitable for real time graphics • Not suitable for teaching first class

  12. Pipeline Model • Close to hardware • Easy to program • Direct implementation of synthetic camera • Image formation by computer is analogous to image formation by camera (or human) • Specify viewer, objects, lights • Let hardware/software form image (via projection)

  13. OpenGL Geometric Pipeline State ApplicationProgram ModelView Transformation Frame Buffer Rasterization Projection Pixels Vertices

  14. Per Vertex Operations & Primitive Assembly Polynomial Evaluator DisplayList Per Fragment Operations Frame Buffer CPU Rasterization Texture Memory Pixel Operations OpenGL Architecture

  15. OpenGL as a Renderer • Geometric primitives • points, lines and polygons • Image Primitives • images and bitmaps • separate pipeline for images and geometry • linked through texture mapping • Rendering depends on state • colors, materials, light sources, etc.

  16. OpenGL and the Windowing System • OpenGL is concerned only with rendering • Window system independent • No input functions • OpenGL must interact with underlying OS and windowing system • Need minimal interface which may be system dependent • Done through additional libraries: AGL, GLX, WGL

  17. GLU and GLUT • GLU (OpenGL Utility Library) • part of OpenGL • NURBS, tessellators, quadric shapes, etc. • GLUT (OpenGL Utility Toolkit) • portable windowing API • not officially part of OpenGL

  18. OpenGL and Related APIs application program OpenGL Motif widget or similar GLUT GLX, AGLor WGL GLU GL X, Win32, Mac O/S software and/or hardware

  19. Preliminaries • Headers Files • #include <GL/gl.h> • #include <GL/glu.h> • #include <GL/glut.h> • Libraries • Enumerated Types • OpenGL defines numerous types for compatibility • GLfloat, GLint, GLenum, etc.

  20. GLUT Basics • Application Structure • Configure and open window • Initialize OpenGL state • Register input callback functions • render • resize • input: keyboard, mouse, etc. • Enter event processing loop

  21. Sample Program void main( int argc, char** argv ) { int mode = GLUT_RGB|GLUT_DOUBLE; glutInitDisplayMode( mode ); glutCreateWindow( argv[0] ); init(); glutDisplayFunc( display ); glutReshapeFunc( resize ); glutKeyboardFunc( key ); glutIdleFunc( idle ); glutMainLoop(); }

  22. OpenGL Initialization • Set up whatever state you’re going to use void init( void ) { glClearColor( 0.0, 0.0, 0.0, 1.0 ); glClearDepth( 1.0 ); glEnable( GL_LIGHT0 ); glEnable( GL_LIGHTING ); glEnable( GL_DEPTH_TEST ); }

  23. GLUT Callback Functions • Routine to call when something happens • window resize or redraw • user input • animation • “Register” callbacks with GLUT glutDisplayFunc( display ); glutIdleFunc( idle ); glutKeyboardFunc( keyboard );

  24. Rendering Callback • Do all of your drawing here glutDisplayFunc( display ); void display( void ) { glClear( GL_COLOR_BUFFER_BIT ); glBegin( GL_TRIANGLE_STRIP ); glVertex3fv( v[0] ); glVertex3fv( v[1] ); glVertex3fv( v[2] ); glVertex3fv( v[3] ); glEnd(); glutSwapBuffers(); }

  25. Idle Callbacks • Use for animation and continuous update glutIdleFunc( idle ); void idle( void ) { t += dt; glutPostRedisplay(); }

  26. User Input Callbacks • Process user input glutKeyboardFunc( keyboard ); void keyboard( char key, int x, int y ) { switch( key ) { case ‘q’ : case ‘Q’ : exit( EXIT_SUCCESS ); break; case ‘r’ : case ‘R’ : rotate = GL_TRUE; break; } }

  27. Elementary Rendering • Geometric Primitives • Managing OpenGL State • OpenGL Buffers

  28. GL_LINES GL_POLYGON GL_LINE_STRIP GL_LINE_LOOP GL_POINTS GL_TRIANGLES GL_QUADS GL_TRIANGLE_FAN GL_TRIANGLE_STRIP GL_QUAD_STRIP OpenGL Geometric Primitives • All geometric primitives are specified by vertices

  29. Simple Example void drawRhombus( GLfloat color[] ) { glBegin( GL_QUADS ); glColor3fv( color ); glVertex2f( 0.0, 0.0 ); glVertex2f( 1.0, 0.0 ); glVertex2f( 1.5, 1.118 ); glVertex2f( 0.5, 1.118 ); glEnd(); }

  30. OpenGL Command Formats glVertex3fv( v ) Data Type Vector Number of components b - byte ub - unsigned byte s - short us - unsigned short i - int ui - unsigned int f - float d - double omit “v” for scalar form glVertex2f( x, y ) 2 - (x,y) 3 - (x,y,z) 4 - (x,y,z,w)

  31. Specifying Geometric Primitives • Primitives are specified using glBegin( primType ); glEnd(); • primType determines how vertices are combined • GLfloat red, greed, blue; • Glfloat coords[3]; • glBegin( primType ); • for ( i = 0; i < nVerts; ++i ) { • glColor3f( red, green, blue ); • glVertex3fv( coords ); • } • glEnd();

  32. Per Vertex Poly. Frag FB Raster CPU DL Texture Pixel OpenGL ColorModels • RGBA or Color Index color index mode Red Green Blue 0 Display 1 1 2 2 4 8 3 ww www 16 24 123 219 74 ww 25 26 www RGBA mode

  33. developed by Nate Robbins Available on web (see references) Shapes Tutorial

  34. Controlling Rendering Appearance • From Wireframe to Texture Mapped

  35. OpenGL’s State Machine • All rendering attributes are encapsulated in the OpenGL State • rendering styles • shading • lighting • texture mapping

  36. Manipulating OpenGL State • Appearance is controlled by current state for each ( primitive to render ) { update OpenGL state render primitive } • Manipulating vertex attributes is most common way to manipulate state glColor*() / glIndex*() glNormal*() glTexCoord*()

  37. Controlling current state • Setting State glPointSize( size ); glLineStipple( repeat, pattern ); glShadeModel( GL_SMOOTH ); • Enabling Features glEnable( GL_LIGHTING ); glDisable( GL_TEXTURE_2D );

  38. Transformations in OpenGL • Modeling • Viewing • orient camera • projection • Animation • Map to screen

  39. Camera Analogy • 3D is just like taking a photograph (lots of photographs!) viewing volume camera model tripod

  40. Camera Analogy and Transformations • Projection transformations • adjust the lens of the camera • Viewing transformations • tripod–define position and orientation of the viewing volume in the world • Modeling transformations • moving the model • Viewport transformations • enlarge or reduce the physical photograph

  41. Coordinate Systems and Transformations • Steps in Forming an Image • specify geometry (world coordinates) • specify camera (camera coordinates) • project (window coordinates) • map to viewport (screen coordinates) • Each step uses transformations • Every transformation is equivalent to a change in coordinate systems (frames)

  42. Affine Transformations • Want transformations which preserve geometry • lines, polygons, quadrics • Affine = line preserving • Rotation, translation, scaling • Projection • Concatenation (composition)

  43. Homogeneous Coordinates • each vertex is a column vector • w is usually 1.0 • all operations are matrix multiplications • directions (directed line segments) can be represented with w = 0.0

  44. 3D Transformations • A vertex is transformed by 4 x 4 matrices • all affine operations are matrix multiplications • all matrices are stored column-major in OpenGL • matrices are always post-multiplied • product of matrix and vector is

  45. Specifying Transformations • Programmer has two styles of specifying transformations • specify matrices (glLoadMatrix, glMultMatrix) • specify operation (glRotate, glOrtho) • Programmer does not have to remember the exact matrices • check appendix of Red Book (Programming Guide)

  46. Programming Transformations • Prior to rendering, view, locate, and orient: • eye/camera position • 3D geometry • Manage the matrices • including matrix stack • Combine (composite) transformations

  47. Per Vertex Poly. Frag FB Raster CPU DL Texture Pixel TransformationPipeline • other calculations here • material è color • shade model (flat) • polygon rendering mode • polygon culling • clipping normalized device eye object clip window v e r t e x Modelview Matrix Projection Matrix Perspective Division Viewport Transform Modelview Projection Modelview l l l

  48. Matrix Operations • Specify Current Matrix Stack glMatrixMode( GL_MODELVIEW or GL_PROJECTION ) • Other Matrix or Stack Operations glLoadIdentity() glPushMatrix() glPopMatrix() • Viewport • usually same as window size • viewport aspect ratio should be same as projection transformation or resulting image may be distorted glViewport( x, y, width, height )

  49. Projection Transformation • Shape of viewing frustum • Perspective projection gluPerspective( fovy, aspect, zNear, zFar ) glFrustum(left,right,bottom,top,zNear,zFar) • Orthographic parallel projection glOrtho(left,right,bottom,top,zNear,zFar) gluOrtho2D( left, right, bottom, top ) • calls glOrtho with z values near zero

  50. Applying Projection Transformations • Typical use (orthographic projection) glMatrixMode( GL_PROJECTION ); glLoadIdentity(); glOrtho( left, right, bottom, top, zNear, zFar );